Michael Cummings

David Reisman • University of South Carolina

Population Genetics and Human Evolution

Chapter 19

Gradients of Genetic Variation in Human Populations Prior to genomics, evolutionary biologists surveyed

populations and cataloged differences in allele frequencies

The distribution of a specific allele as a gradient across continents is called a cline.

The gradient of an allele reflects waves of migration

This data provides clues to the origin of humans—the older populations have had more generations to evolve and thus have greater diversity of alleles.

http://www.genome.gov/25019968

Are There Human Races?

Race—term used in the 19th century to describe phenotypic differences among populations.

Studies of variations in proteins, microsatellites regions of DNA, and expressed genes show more genetic variation within populations than between populations

Main Conclusion: There is no clear genetic basis for dividing our species into races

Variations used in Genomic Studies

Table 19-5, p. 428

Conclusions of Population Studies

Based on studies from the 1990’s to the present… There is very little genetic variation in the human

genome Variation in the human genome is continuously

distributed Most genetic variation is widely shared, but a small

amount is geographically clustered Some genetic analyses can sometimes allow the

identification of an individuals continent of ancestry (see Genographic Project slide later)

Fig. 19-9, p. 429

Australian Caucasoid

Human genetic

variationNorth American African

‘‘Race’’ 2 ‘‘Race’’ 1

South American African‘‘Race’’ 3

Indian Mainland Asian

Each circle in the center represents genetic variation within a population defined as a race. The variations overlap greatly as shown by the dark grey in the center. Few to no genetic differences belong to a single racial group.

19.6 The Evolutionary History and Spread of Our Species (Homo sapiens)

A combination of anthropology, paleontology, archaeology, and genetics is being used to reconstruct the dispersal of human populations around the globe

Our evolutionary history begins with the hominoid lineage about 25 million years ago

Hominoid – superfamily of primates, including apes and humans

Images from http://christpantokrator.blogspot.com/2011/06/human-evolution-diagrams.html

Variations on Rudolph Zallinger’s March of Progress

Evolution is not a linear morphing of one type of organism into another – a common misunderstanding

Rather, evolution is branching from a shared ancestor

http://pewresearch.org/pubs/1105/darwin-debate-religion-evolutionhttp://tolweb.org/onlinecontributors/app

Genome-based evolutionary relationships among hominoids

Fig. 19-10, p. 430

7 m.y.a. chimps and humans had a common ancestor

After human line split from the chimps, three different species groups appeared.

Collectively known as hominins• Australopithecines• Paranthropus• Homo – our ancestral group

Early humans emerged ~5 million years ago

Fig. 19-11, p. 430

Homo floresiensisHomo rudolfensisAustralopithecus

anamensis Homo habilis Homo sapiens

Australopithecus africanus Homo erectus

Australopithecus afarensis Australopithecus garhi

Homo neanderthalensisParanthropus

aethiopicus Paranthropus robustus

Paranthropus boisei

4 3 2 1 PresentTime (millions of years ago)

Estimates of the dates of origin and extinction of the three main groups of hominins (green, blue, and orange). The australopithecines split into two groups about 2.5 to 2.7 million years ago.

Two Theories Differ on How and Where Homo sapiens Originated

Hypothesis 1: Modern Homo sapiens arose once, in one place, from its ancestral species. Members of this species then migrated from there to all parts of the globe. (Out-of-Africa hypothesis)

Hypothesis 2: Modern Homo sapiens arose in a number of different locations from similar ancestral populations at roughly the same time. The hypothesis assumes that although the populations were in different locations, they did interbreed and exchange genes. (Multiregional hypothesis)

Genetic evidence supports model #1.

Humans Have Spread Across the World

Available evidence suggests that• H. sapiens emigrated from Africa about 137,000 years

ago• H. sapiens spread through Southeast Asia and

Australia 40,000 to 60,000 years ago• H. sapiens replaced Neanderthals in Europe 40,000

to 50,000 years ago• North America and South America were populated in

waves 15,000 to 30,000 years ago

Fig. 19-13, p. 432

European population Origin: 40,000 to 50,000 years ago

Asian population Origin: 50,000 to 70,000 years ago

New World population Origin: 20,000 to 30,000 years ago

Immigration from Africa About 137,000 years ago; 200 to 500 or more individuals

African populations Origin: 130,000 to 170,000 years ago Population: 23,000 to 45,000

Australo-Melanesian population Origin: 40,000 to 60,000 years ago

The origin and spread of modern H. sapiens, reconstructed from genetic and fossil evidence.

*All human populations are derived from African populations**Colors correspond to major continental regions.

Fig. 19-12, p. 431

Although separated for about 7 million years, analysis of human and chimp genomes shows many similarities and subtle differences• The DNA sequences are 98.8% identical• Variations due to insertions, deletions and duplications

differ, ultimately change gene dosage• There is a 1% difference in coding sequence of genes• Phenotypic differences cannot be explained by differences

in coding sequences• The important differences may involve gene regulation and

genes that control body structure

19.7 Genomics and Human Evolution

Neanderthals are not Closely Related to Humans H. neanderthalensis lived in the Middle East, Asia,

and Europe 300,000 to 30,000 years ago.

Analysis of DNA recovered from Neanderthal remains clearly show that humans did not descend from them. (Neanderthal genome sequenced in 2010.

Some interbreeding did occur most likely in the Middle East before humans expanded into Europe and Asia

1-4% of genes carried by non-africans are from Neanderthals

Fig. 19-15, p. 434

–440,000 to 270,000 y. a. Split of ancestral human

and Neanderthal populations

–41,000 y. a. Earliest modern

humans in Europe

–706,000 y. a. Coalescence of human

and Neanderthal reference sequences

–195,000 y. a. Earliest known anatomically

modern humans

–28,000 y. a. Most recent known

Neanderthal remains

Modern human

NeanderthalGenomic data

Fossil data

Genomic and fossil evidence has been used to estimate the time of divergence of human and Neanderthal lines relative to landmark events evolution. Genomic analyses trace evolution back much farther than fossils can.

DNA as a Molecular Clock

The rate of mutation between two DNA sequences can be used as a clock to provide a relative measure of time since divergence from a common ancestor

Assumes that mutation rate is constant Can be calibrated by comparison to the fossil

record

The Genetic Revolution: Tracing Ancient Migrations How can we map out events that occurred thousands of

years ago?• The answers are written in the genomes of present day

populations• Genetic markers on the Y chromosome are passed

from father to son• Markers in mitochondrial DNA are passed from mother

to all offspring• These markers do not undergo recombination in

meiosis—individuals carry these markers to new locations as the migrate• The Genographic Project:

https://genographic.nationalgeographic.com/genographic/index.html

An Important Gene in Language Development The gene is called FOXP2 It is present in chimpanzees, modern humans, and

Neanderthals Genes and pathways controlled by FOXP2 differ

among these groups

http://www.physorg.com/news188139245.html With the help of a little singing bird, Penn State physicists are gaining insight into how the human brain functions, which may lead to a better understanding of complex vocal behavior, human speech production and ultimately, speech disorders and related diseases.